The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

As with all shorthand properties, border-bottom always sets the values of all of the properties that it can set, even if they are not specified. It sets those that are not specified to their default values. This means that ...

border-bottom-style: dotted;
border-bottom: thick green;

... is actually the same as ...

border-bottom-style: dotted;
border-bottom: none thick green;

... and the value of border-bottom-style given before border-bottom is ignored. Since the default value of border-bottom-style is none, not specifying the border-style part results in no border.